By exploring how creatures in nature are able to fly by flapping their
wings, Virginia Tech researchers hope to apply that knowledge toward designing
small flying vehicles known as "micro air vehicles" with flapping
wings. More than 1,000 species of bats have hand membrane wings, meaning that their fingers are essentially "webbed" and connected by a flexible membrane. But understanding how bats use their wings to manipulate the air around them is extremely challenging -- primarily because both experimental measurements on live creatures and the related computer analysis are quite complex.

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In Virginia Tech's study of fruit bat wings, the researchers used
experimental measurements of the movements of the bats' wings in real flight,
and then used analysis software to see the direct relationship between wing
motion and airflow around the bat wing. They report their findings in the
journal Physics of Fluids.

"Bats have different wing shapes and sizes, depending on their
evolutionary function. Typically, bats are very agile and can change their
flight path very quickly -- showing high maneuverability for midflight prey
capture, so it's of interest to know how they do this," explained Danesh
Tafti, the William S. Cross professor in the Department of Mechanical
Engineering and director of the High Performance Computational Fluid Thermal
Science and Engineering Lab at Virginia Tech.

To give you an idea of the size of a fruit bat, it weighs roughly 30
grams and a single fully extended wing is about 17 x 9 cm in length, according
to Tafti.

Among the biggest surprises in store for the researchers was how bat
wings manipulated the wing motion with correct timing to maximize the forces
generated by the wing. "It distorts its wing shape and size continuously
during flapping," Tafti noted.

For example, it increases the area of the wing by about 30 percent to
maximize favorable forces during the downward movement of the wing, and it
decreases the area by a similar amount on the way up to minimize unfavorable
forces. The force coefficients generated by the wing are "about two to
three times greater than a static airfoil wing used for large airplanes,"
said Kamal Viswanath, a co-author who was a graduate research assistant working
with Tafti when the work was performed and is now a research engineer at the
U.S. Naval Research Lab's Laboratories for Computational Physics and Fluid
Dynamics.

This study was just an initial step in the researchers' work.
"Next, we'd like to explore deconstructing the seemingly complex motion of
the bat wing into simpler motions, which is necessary to make a bat-inspired
flying robot," said Viswanath.